Nucleation of aluminum nitride on a silicon substrate using an ammonia preflow

ABSTRACT

A method of making an aluminum nitride (AlN) buffer layer over a silicon wafer for a light emitting diode (LED) includes preflowing a first amount of ammonia that is sufficient to deposit nitrogen atoms on the surface of a silicon wafer without forming SiNx, before flowing trimethylaluminum and then a subsequent amount of ammonia through the chamber.

RELATED APPLICATIONS

The present application is a divisional of co-pending, commonlyassigned, patent application Ser. No. 13/190,420, entitled “NUCLEATIONOF ALUMINUM NITRIDE ON A SILICON SUBSTRATE USING AN AMMONIA PREFLOW,”filed Jul. 25, 2011, which was abandoned on Sep. 17, 2014.

TECHNICAL FIELD

The present invention relates generally to methods of growing galliumnitride on silicon.

BACKGROUND INFORMATION

Thin films of Group III nitride, such as gallium nitride (GaN), are usedin the production of efficient optoelectronic light emitters.Conventionally, GaN has been grown directly on sapphire substrates(Al₂O₃). The GaN is grown in thin layers as opposed to in a singlethree-dimensional growth mode in order to achieve a high qualitycrystalline structure of the epitaxial growth. Growing the epitaxiallayers of GaN on silicon as opposed to on sapphire offers considerablecost savings because of the economies of scale from the large productionof silicon for the semiconductor industry. A large amount of equipmentfor the production of crystalline silicon has already been depreciated.That equipment can now be used in the production of light emittingdiodes (LEDs).

Current attempts to grow high quality epitaxial layers of GaN on siliconsubstrates, however, have not been entirely successful. Because of thegreat difference between the lattice constants and thermal expansioncoefficients, of GaN and silicon, GaN is not well suited for epitaxialgrowth directly on a silicon substrate. GaN epilayers often crack uponcooling to room temperature because even at growth temperatures above1000° C. the lattice constant of GaN is much smaller than that ofcrystalline silicon. In addition, GaN has a much larger coefficient ofthermal expansion than does silicon. So as the layers of GaN grown onsilicon at high temperature cool to room temperature, the smallerlattice distance of the GaN crystals relative to the silicon crystalsbecomes even more pronounced. The GaN layers deposited directly onsilicon are subjected to even more tensile stress as they cool and caneven cause the underlying silicon substrate to bow.

Consequently, attempts have been made to grow buffer layers between thesilicon substrate and the epitaxial GaN layers in order to compensatefor the differing lattice constants and thermal expansion coefficientsof GaN and silicon. For example, buffer layers of AlN, AlGaN and AlGaINhave been grown between the silicon substrate and the GaN layers.

The quality of the epitaxial GaN layers, however, that can be grown overexisting buffer layers has been poor. Current methods of forming bufferlayers of AlN and AlGaN have resulted in epitaxial growth of GaN layersthat contain structural defects such as discontinuities, dislocationsand faults. These defects degrade the morphology and optical propertiesof the GaN layers, rendering the GaN layers unsuitable for use in highqualify LEDs.

A method is sought for growing buffer layers on a silicon substrate thatallows high quality epitaxial GaN layers with fewer structural defectsto be grown over the buffer layers.

SUMMARY

A silicon wafer used in manufacturing crystalline gallium nitride (GaN)for light emitting diodes (LEDs) includes a silicon substrate, a bufferlayer, of aluminum nitride (AlN), a second buffer layer of aluminumgallium nitride (Al_(x)Ga_(1-x)N), and an upper layer of GaN. Thesilicon wafer has a diameter of at least 200 millimeters and anSi(111)1×1 surface (as opposed to a Si(111)7×7 reconstructed surface).The AlN buffer layer overlies the Si(111) surface of the substrate andis between 205 to 250 nanometers thick. The second buffer layer ofaluminum gallium nitride is disposed between the buffer layer ofaluminum nitride and the upper layer of gallium nitride.

Across the entire wafer substantially no aluminum atoms of the AlN arepresent in a bottom most plane of atoms of the AlN buffer layer, andacross the entire wafer substantially only nitrogen atoms of the AlNbuffer layer are present in the bottom most plane of atoms of the AlN.Thus, the AlN buffer layer has a single polarity. The silicon and AlNare oriented as AlN<0001>∥Si<111>. No amount of metallic aluminum isdisposed between the silicon substrate and the AlN buffer layer. Inaddition, no layer of SiN_(x) is present between the silicon substrateand the AlN buffer layer.

A method of making an AlN buffer layer includes preflowing a first smallamount of ammonia before flowing trimethylaluminum in order to formsingle polarity AlN. The crystallinity of the AlN buffer layer isinfluenced by the quality of the initial nucleation layer of AlN and thenature of the atomic bonding between the AlN and the silicon(111)surface. Because of the ammonia preflow step, the initial nucleationlayer of AlN begins to grow with only nitrogen atoms bonded to thesilicon(111) surface over the entire surface of the silicon wafer.

In a first cleaning step, a substrate of silicon (Si) is heated to atemperature above 950° C. in a reaction chamber of a metal-organicchemical vapor deposition (MOCVD) device. Then hydrogen (H₂) is flowedthrough the chamber in an amount between 106 and 118 cubic centimetersof hydrogen per minute over each square centimeter of the surface of thesilicon substrate. In one aspect, the temperature in the chamber duringthe flowing of hydrogen is above 1100° C.

In the ammonia preflow step, a first amount of ammonia is flowed throughthe reaction chamber while the hydrogen is still flowing through thechamber. The first amount of ammonia is less than 0.01% by volume of thehydrogen flowing through the chamber. The first amount of ammonia doesnot exceed 0.006 cubic centimeters per minute over each squarecentimeter of the surface of the silicon substrate. The ammonia preflowstep is performed for between thirty seconds to three minutes. Thetemperature in the chamber during the ammonia preflow step is between1000° C. and 1050° C.

Then, trimethylaluminum (Al₂(CH₃)₆) is flowed through the chamber whilethe hydrogen and first amount of ammonia are still flowing through thechamber. The trimethylaluminum is flowed through the chamber for betweenten to twenty minutes in an amount of about ninety micromoles perminute.

Then a subsequent amount of ammonia is flowed through the chamber whilethe trimethylaluminum is still flowing through the chamber. Thesubsequent amount of ammonia is greater than 0.002% by volume of thehydrogen, flowing through the chamber. In one aspect, the subsequentamount of ammonia flowed through the chamber was just under 5% of thetotal amount of hydrogen, ammonia and trimethylaluminum.

Further details and embodiments and techniques are described in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 is a cross-sectional view showing the growth of a single crystalGaN film on buffer layers over a silicon substrate.

FIG. 2 is a flowchart of a method for growing an initial nucleationlayer of AlN on a silicon substrate.

FIG. 3A shows a model of the crystal structure of silicon.

FIG. 3B is a diagram of the silicon atoms along an Si(111)1×1 surface ofsilicon.

FIG. 4 is a graph of gas flows of hydrogen, ammonia andtrimethylaluminum through a reaction chamber during the method of FIG.2.

FIG. 5 shows a model of the crystal structure of wurtzite aluminumnitride.

FIG. 6 is a diagram of the crystal structure of the aluminum hexagons inthe C-plane of AlN superimposed over the crystal structure of siliconalong the Si(111)1×1 surface.

FIG. 7 is a diagram of the crystal structures of a silicon substrate andan AlN nucleation layer viewed perpendicular to the Si(111) surface.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 is a schematic diagram showing the growth of a single crystal GaNfilm 10 on buffer layers over a silicon substrate 11. A buffer layer ofaluminum nitride (AlN) 12 is first grown on the silicon substrate 11.Then higher buffer of layers of aluminum gallium nitride(Al_(x)Ga_(1-x)N) 13 are grown over the AlN layer 12. Finally, the GaNlayer 10 is grown over the top layer of aluminum gallium nitride 13. Insome embodiments, the GaN layer includes several sublayers. The bufferlayer of AlN 12 is made up of a lower initial nucleation layer 14 andthicker upper layers 15.

There are several reasons for first growing buffer layers on a siliconsubstrate before the gallium nitride (GaN) layer 10 is grown. First,meltback etching of the silicon substrate 11 by gallium occurs if thegallium is allowed to react directly with the silicon substrate. Thereaction between gallium and silicon results in poor crystal quality andmorphology of the GaN layer 10. Second, SiN_(x) can be formed as GaN isdeposited on silicon, which leads to a 3-dimensional growth of GaNcrystals instead of a 2-dimensional growth over the entire surface ofthe silicon substrate before the GaN layer thickens. Three-dimensionalcrystal growth leads to lower quality GaN layer than does 2-dimensionalcrystal growth. Third, the lattice mismatch between GaN and crystallinesilicon causes a large tensile strain on the GaN layer at the interfacewith the silicon. The lattice mismatch at room temperature between GaNand the hexagonal surface orientation of silicon Si(111) is about 16.9%.Fourth, the in-plane thermal expansion coefficients of GaN on Si(111)vary widely (5.59×10⁻⁶K⁻¹ for GaN and 2.6×10⁻⁶K⁻¹ for Si). Thedifference in thermal expansion coefficients can result in cracking ofthe GaN layer upon cooling from the growth temperature to roomtemperature.

To solve the problems caused by growing GaN directly on silicon, bufferlayers are typically deposited between the silicon and the GaN. Forexample, a nucleation layer of AlN 12 can first be grown on the siliconsubstrate 11, followed by the additional buffer layers 13 of aluminumgallium nitride (Al_(x)Ga_(1-x)N). The AlN nucleation layer and theother buffer layers solve the four problems described above. First, theAlN buffer layer 12 does not allow gallium to come into contact with thesilicon substrate 11. Second, GaN is not grown directly on the siliconsubstrate, so the formation of amorphous SiN_(x) can be prevented on thesurface of the silicon, which would otherwise degrade the crystalformation of the GaN. Third, the lattice mismatch between GaN andSi(111) is compensated by the smaller lattice constant of AlN thatapplies compressive stress to the GaN to counteract the tensile stressof GaN from the underlying Si(111). Fourth, the smaller lattice constantof AlN compensates for the greater proportionate shrinking of the GaNcrystal compared to the silicon crystal as both cool from the growthtemperature to room temperature.

The quality of the GaN layer and other epitaxial layers, however, isalso dependent on the quality of the AlN nucleation layer 12.Consequently, simply growing a layer of AlN to solve the aforementionedfour problems that result from growing directly on silicon will notnecessarily result in high-quality GaN. The properties of the aluminumnitride nucleation layer 12, such as its dislocation density and surfacemorphology, are critical in influencing the properties of the higherepitaxial layers. The AlN layer 12 acts as a crystallographic templatefor the higher buffer layers and ultimately for the GaN layer 10. Theproperties of the AlN layer 12, in turn, are determined in large part bythe conditions under which the growth of the AlN is initiated and by howthe silicon substrate 11 is treated prior to growth of the AlN.

It would seem that growing an AlN layer that has a low dislocationdensity over Si(111) would be hindered by the 23.4% lattice mismatchbetween AlN and Si(111). The distance between silicon atoms in the (111)plane of a silicon crystal is 3.840 angstroms, whereas the distancebetween aluminum atoms or between nitrogen atoms in the C-plane ofwurtzite AlN is 3.112 angstroms. Smooth interface morphology between AlNand Si(111) can be achieved, however, due to the lattice coincidencebetween the (111) plane of silicon and the C-plane of AlN which permitsthe relaxation of the crystal stress at regular intervals of misfitdislocation at the AlN/Si interface. Achieving the same type of misfitdislocations at regular intervals is critical to obtaining a smoothinterface morphology.

A method is disclosed for beginning the growth of AlN that results in asmooth interface between AlN and Si(111). The method grows a singlepolarity buffer layer of AlN having a low dislocation density. Thesubsequent buffer layers grown over the AlN buffer layer retain the highquality crystalline form and enable higher quality GaN and otherepitaxial layers to be grown over the buffer layers.

FIG. 2 is a flowchart illustrating steps of a method 16 for growing ahigh-quality AlN nucleation layer on a silicon substrate. In a firststep 17, a substrate of Silicon (Si) is heated to a temperature above950° C. in a chamber. In one aspect, the substrate was heated to atemperature of 1140° C. in the reaction chamber of a metal-organicchemical vapor deposition (MOCVD) system (also called a metal-organicvapor phase epitaxy system). The substrate was in the form of three8-inch silicon wafers cut along the (111) plane. The three wafers wereplaced on a wafer receptacle having a diameter of 465 millimeters.

FIG. 3A is a diagram illustrating the crystal structure 23 of silicon.The (111) plane along which the three silicon wafers are cleaved is theplane that intersects the silicon atoms A, C and F in FIG. 3A. Thesilicon atoms A, F and 24 define the (010) plane, in which the siliconatoms form a square format. The distance between silicon atoms atadjacent corners of the squares of the identical (100), (010) and (001)planes is 5.431 angstroms. There is a shorter distance, however, betweenadjacent silicon atoms of the hexagonal format of the (111) plane. Forexample, the distance between atoms A and B in the (111) plane is 3.840angstroms. This shorter distance between atoms in the hexagonal formatof Si(111) better matches the distance between nitrogen atoms in thehexagonal format along the C-plane of AlN. FIG. 3B illustrates the atomsA-F of FIG. A in which the (111) plane coincides with the plane of thepage. The surface of the three silicon wafers after first step 17 has aSi(111)1×1 structure 25 as shown in FIG. 3B as opposed to thedimer-adatom-stacking (DAS) fault structure of the Si(111)7×7reconstruction. The more-stable, faceted Si(111)7×7 surface structurebreaks down to the regular Si(111)1×1 hexagonal structure 25 as thesilicon substrates are heated above about 850° C.

In step 18, hydrogen is flowed through the chamber in order to removeSiO₂ from the wafers and generally to clean the surface of the siliconsubstrate. Between 106 and 118 cubic centimeters of hydrogen per minuteflow over each square centimeter of the surface of the substrate. In oneaspect, 180-200 liters of hydrogen per minute was flowed through thechamber. The silicon substrate was baked in the hydrogen flow at 1140°C. for about fifteen minutes to remove the native oxide. Then, thetemperature in the chamber was lowered to about 1020° C.

In step 19, a first amount of ammonia (NH3) is flowed through thechamber while the hydrogen is still flowing through the chamber. Thefirst amount of ammonia is less than 0.01% of by volume of the hydrogenflowing through the chamber. The first amount of ammonia is flowedthrough the chamber for between thirty seconds to three minutes. In oneaspect, less than ten cubic centimeters of ammonia per minute was flowedover the 465-mm wafer receptacle. Thus, less than 0.00588 cubiccentimeters per minute of ammonia flowed over each square centimeter ofthe surface of the silicon substrate. At 1020° C., the first amount ofammonia is insufficient to form a layer of SiN_(x) over the surface ofthe silicon substrate. The first amount of ammonia is, however,sufficient to form a small number of Si—N bonds on the Si(111)1×1surface.

In step 20, trimethylaluminum (Al₂(CH₃)₆) is flowed through the chamberwhile the hydrogen is still flowing through the chamber. Thetrimethylaluminum flows through the chamber in an amount of about ninetymicromoles per minute. In one aspect, ninety micromoles per minute oftrimethylaluminum flowed through the chamber for between ten to twentyminutes.

In step 21, a subsequent amount of ammonia is flowed through the chamberwhile the trimethylaluminum is still flowing through the chamber. Thesubsequent amount of ammonia is greater than 0.002% by volume of thehydrogen flowing through the chamber. In one aspect, the subsequentamount of ammonia flowed through the chamber was just under 5% of thetotal amount of hydrogen, ammonia and trimethylaluminum. When thesubsequent amount of ammonia was flowed through the chamber at justunder 5% of the total flow for about fifteen minutes, an initialnucleation layer 14 of aluminum nitride (AlN) grew to a thickness ofbetween 25-50 nanometers. The crystallinity of the AlN buffer layer 12is related to the quality of the initial nucleation layer 14 and thenature of the atomic bonding between the silicon(111) surface and theAlN. Because of the ammonia preflow in step 19, the initial nucleationlayer 14 begins to grow with only nitrogen atoms bonded to thesilicon(111) surface over the entire surface of the 8-inch wafers.

In a step 22, the flow of trimethylaluminum is increased to about 180micromoles per minute, and the temperature in the chamber is increasedto about 1120° C. The buffer layer of AlN is grown an additional 180-200nanometers to a total thickness of 205-250 nanometers under theincreased flow of trimethylaluminum.

FIG. 4 is a graph that represents the flows of hydrogen, ammonia andtrimethylaluminum through the reaction chamber during the cleaning,preflow, initial growth and thicker growth stages of the formation ofthe AlN buffer layer. In other embodiments, the additional 180-200nanometers of AlN is grown in multiple stages of highertrimethylaluminum concentration as opposed to in one step.

The initial nucleation layer 14 of AlN first starts to form when thetrimethylaluminum starts flowing through the chamber in step 20 andbefore the subsequent amount of ammonia is flowed through the chamber instep 21. Consequently, a very small amount of nitrogen is present on theSi(111)1×1 surface of the silicon substrate 11 before the aluminum fromthe trimethylaluminum comes into contact with the substrate surface. Asthe first seed crystals of AlN are formed, the aluminum atoms form bondsto the nitrogen atoms that are present on the Si(111)1×1 surface asopposed to directly with silicon atoms on the substrate surface. Thenitrogen from the ammonia preflow step ensures that the polarity of thealternating layers of aluminum and nitrogen in the AlN crystals that areformed across the entire silicon wafer will have a nitrogen layer facingthe silicon substrate and an aluminum layer on top.

FIG. 5 is a diagram illustrating the crystal structure 26 of wurtzitealuminum nitride (AlN). The smaller spheres represent aluminum atoms 27,and the larger spheres represent nitrogen atoms 28. The C-plane of theAlN crystal intersects all six aluminum atoms that form a hexagon on thetop surface of the crystal. The distance between adjacent aluminum atomsaround the hexagon in the C-plane is 3.112 angstroms. The distancebetween adjacent nitrogen atoms around the middle hexagon is thereforealso 3.112 angstroms. The hexagons of nitrogen and aluminum along theC-plane of AlN approximately match the hexagonal format of silicon atomson the Si(111)1×1 surface of the silicon substrate.

FIG. 6 is a diagram of the crystal structure of the aluminum hexagons inthe C-plane of AlN superimposed over the crystal structure of thesilicon substrate on the Si(111)1×1 surface. Because the distancebetween atoms around the silicon hexagon is 3.840 angstroms and thedistance between the atoms around the hexagons of AlN is 3.112angstroms, the lattice distance of a silicon cell unit is 6.652angstroms and the lattice distance of an AlN cell unit is 5.390angstroms. Thus, there is a 23.4% lattice mismatch. However, the widthof five AlN cell units (26.95 angstroms) approximately matches the widthof four silicon cell units (26.61 angstroms), as illustrated in FIG. 6.Every fifth AlN cell unit can bond well to every fourth silicon cellunit.

FIG. 7 is a diagram of the crystal structures of the silicon substrateand AlN viewed perpendicular to the Si(111)1×1 surface and the C-planeof AlN. FIG. 7 illustrates how each fifth cell unit of the AlN crystalapproximately matches each fourth cell unit on the Si(111)1×1 surface.The regular mismatch between surface structures of AlN and siliconallows a crystal of AlN to be grown with a low dislocation density.

The dislocation density of the AlN buffet layer is considerably higher,however, if not all islands of AlN crystals that begin to form on theSi(111)1×1 surface have the same polarity. If some islands of AlNcrystals form with nitrogen atoms bonded to the silicon while otherislands of AlN crystals form with aluminum atoms bonded to the silicon,then discontinuities and stacking faults will form where the islands ofcrystals having opposite polarity grow together. FIG. 7 shows that theinitial nucleation layer of AlN formed using method 16 has only nitrogenatoms bonded to the Si(111)1×1 surface. Across the entire substratewafer substantially only nitrogen atoms of the aluminum nitride arepresent in the bottom most plane of atoms of the aluminum nitride.Because the initial nucleation layer of AlN formed using method 16 has asingle polarity, it was possible to grow a GaN layer above thenucleation layer that had a dislocation density of less that 2×10⁹ cm⁻².

Some prior methods of growing an AlN buffer layer begin by depositing ametallic Al layer on the surface of the silicon substrate before the AlNis grown in order to prevent the formation of amorphous SiN_(x). Thepresence of aluminum atoms on surface of the silicon substrate probablycauses at least some of the islands of AlN crystals to form withaluminum as the bottom most plane of atoms of the aluminum nitride.Because these prior art methods do not prevent at least some nitrogenatoms from bonding to the silicon substrate (not necessarily asamorphous SiN_(x)), some islands of AlN crystals form with nitrogen asthe bottom most plane of atoms of the aluminum nitride, and theresulting AlN layer has mixed polarity. Method 16, on the other hand,allows a single polarity material to be grown.

Thus, method 16 can be used to manufacture a wafer of silicon substrateover which an AlN buffer layer, AlGaN layers and finally an upper GaNlayer are grown. The silicon substrate has an Si(111) surface, whichconverts from a 7×7 structure to a 1×1 structure when the silicon isheated above about 850° C. The AlN buffer layer is a means forcompensating for the lattice mismatch between the GaN and the Si(111)surface of the silicon substrate so as to enable the upper GaN layer togrow under reduced stress. The AlGaN on top of the AlN buffer layer is abetter lattice match for GaN than in silicon. The silicon substrate is awafer with a diameter of at least 200 millimeters, such as an 8-inchwafer. The AlN buffer layer overlies the Si(111) surface of thesubstrate and is oriented as AlN<0001>∥Si<111>. The upper layer of GaNis grown on the AlGaN layers over the AlN buffer layer. Across theentire wafer substantially no aluminum atoms of the aluminum nitride arepresent in a bottom most plane of atoms of the aluminum nitride, andacross the entire wafer substantially only nitrogen atoms of thealuminum nitride are present in the bottom most plane of atoms of thealuminum nitride. Thus, across the entire wafer substantially onlynitrogen atoms of the AlN buffer layer form bonds to the Si(111)surface. There is neither metallic aluminum nor any layer of SiN_(x)present between the silicon substrate and the AlN buffer layer.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. A method performed in sequential ordercomprising: (a) first, heating a substrate of silicon (Si) in a chamber,wherein the chamber's temperature is above 950° C.; (b) second, changingthe chamber's temperature from the above 950° C. to a second temperatureand flowing hydrogen (H₂) into the chamber; (c) third, forming a layeron the substrate of Si having no nitride (N) at least by: changing thechamber's temperature from the second temperature to a thirdtemperature, and flowing a first amount of ammonia (NH₃) into thechamber while the hydrogen is still flowing into the chamber, whereinthe first amount of ammonia is less than 0.01% by volume of the hydrogenflowing into the chamber (d) fourth, flowing trimethylaluminum(Al₂(CH₃)₆) into the chamber while the hydrogen is still flowing intothe chamber; and (e) fifth, changing the chamber's temperature from thethird temperature to a fourth temperature and flowing a subsequentamount of ammonia into the chamber while the trimethylaluminum is stillflowing into the chamber, wherein the subsequent amount of ammonia isgreater than 0.002% by volume of the hydrogen flowing into the chamber,wherein the first temperature, second temperature, and third temperatureare different from each other.
 2. The method of claim 1, wherein theflowing the first amount of ammonia into the chamber is performed forbetween thirty seconds to three minutes.
 3. The method of claim 1,wherein the substrate is a wafer having a surface, and wherein the firstamount of ammonia does not exceed 0.006 cubic centimeters per minuteover each square centimeter of the surface of the substrate.
 4. Themethod of claim 1, wherein the substrate is a wafer having a surface,and wherein the flowing the hydrogen into the chamber is performed byflowing between 106 and 118 cubic centimeters of hydrogen per minuteover each square centimeter of the surface of the substrate.
 5. Themethod of claim 1, wherein the flowing the trimethylaluminum into thechamber is performed for between ten to twenty minutes.
 6. The method ofclaim 1, wherein trimethylaluminum flows into the chamber in an amountof about ninety micromoles per minute.
 7. The method of claim 1, whereinthe second temperature in the chamber during the flowing of hydrogen in(b) is above 1100° C., and wherein the third temperature in the chamberduring the flowing of the first amount of ammonia in (c) is between1000° C. and 1050° C.
 8. The method of claim 1, wherein the thirdtemperature in the chamber during the flowing of the first amount ofammonia in (c) is the same as the fourth temperature in the chamberduring the flowing of the subsequent amount of ammonia in (e).
 9. Themethod of claim 1, further comprising: (f) increasing the flow of thetrimethylaluminum at a fifth temperature.
 10. The method of claim 9,wherein the third temperature in the chamber during the flowing of thefirst amount of ammonia in (c) is the same as the fourth temperature inthe chamber during the flowing of the subsequent amount of ammonia in(e), and lower than the fifth temperature in the chamber during theincreased flow of the trimethylaluminum in (f).
 11. A method, performedin sequential order, of manufacturing a semiconductor device, the methodcomprising: first, providing a silicon substrate in a chamber; second,cleaning a surface of the silicon substrate with a flow of hydrogen inthe chamber; third, forming a layer on the substrate of Si having nonitride (N) at least by flowing a first amount of ammonia in the chamberwhile the hydrogen is still flowing into the chamber, wherein the firstamount of ammonia forms nitrogen-silicon bonds without forming SiN_(x)at the surface of the silicon substrate; fourth, flowingtrimethylaluminum (Al₂(CH₃)₆) in into the chamber while the hydrogen isstill flowing into the chamber; and fifth, flowing a second amount ofammonia into the chamber, wherein the second amount of ammonia isgreater than 0.002% by volume of the hydrogen flowing into the chamber.12. The method of claim 11, wherein the first amount of ammonia is lessthan 0.01% by volume of the hydrogen flowing in the chamber.
 13. Themethod of claim 11, wherein the second amount of ammonia is greater than0.002% by volume of the hydrogen flowing in the chamber.
 14. The methodof claim 11, wherein the second amount of ammonia is less than 5% of thetotal amount of hydrogen, ammonia, and trimethylaluminum simultaneouslyflowing in the chamber.
 15. The method of claim 11, wherein the flow ofhydrogen in the chamber is 180 to 200 liters/min.
 16. The method ofclaim 11, wherein a temperature in the chamber during the flowing of thefirst amount ammonia and second amount of ammonia is between 1000° C.and 1050° C., and a temperature in the chamber during the flowing of thetrimethylaluminum is 1120° C.
 17. The method of claim 11, wherein thetrimethylaluminum is about 90 micromoles per minute.